Most Linux systems use one of two boot loaders, the Linux Loader (LILO) or Grub. These boot loaders control your boot images and determine what kernel is booted when the system is started or rebooted. They are loaded after your Basic Input/Output System (BIOS) has initialized your system and generally wait a set period of time (generally between 10 and 30 seconds, but you can override this) for you to select a kernel to boot into; if you have not intervened, then they default to a specified kernel and boot into that.

I recommend you do not have too many kernel versions available to boot into, especially older versions of kernels. Many people leave older kernels on their systems and in their boot loader menus. The risk exists that you, or an attacker, could boot into an older kernel with
a security vulnerability that could allow an attacker to compromise your system. Clean up when you perform kernel upgrades. I recommend leaving the current and previous versions of the kernel on the system (unless, of course, you have upgraded from the previous kernel to correct a security vulnerability).

Both boot loaders, LILO and Grub, are inherently insecure if your attacker has physical access to your system. For example, by default both LILO and Grub will allow you to boot into single-user mode. In single-user mode you have root privileges without having to enter the root password. Additionally, you can enter a variety of other parameters on both the boot loader’s command lines that can provide an attacker with opportunities to compromise your system.
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